This invention relates to electronic switching circuits and, more particularly, to a circuit for selectively triggering thyristors for controlling A.C. circuits.
Thyristors, such as SCR's and triacs, are becoming ever more popular as replacements for mechanical switches and electro-mechanical relays in the control of A.C. circuits. New uses and more sophisticated applications for thyristors as A.C. power control devices are constantly being developed.
For the past few years there have been different types of "relays" commercially available that utilize thyristors as the active switching element. Recently attention has been focused on maintaining positive isolation between the input and the output of these relays and thereby eliminating feedback normally resulting from their use.
The best input/output isolation available today is in the "optically coupled" relays in which the input circuit comprises a light emitting diode and the trigger circuit for the thyristor comprises a photo responsive component that is optically coupled to the light emitting component. Many of these optically coupled relays adequately provide the much sought after isolation and are being widely utilized by design engineers.
Another advantage provided by the use of a thyristor as a switching element in a relay is derived from the rapidity with which thyristors change state. Initiation of conduction in thyristors is rapid enough that, by employing proper trigger circuitry, thyristors will become conductive only at controlled, preselected times within the cycle of the A.C. voltage to be controlled. Thus, thyristors can be made to turn on only near the A.C. crossovers of the supply voltage. Consequently, problems associated with voltage and current transients incurred following switching are alleviated.
As the aforementioned functional features were designed into solid state relays, the circuits of the relays became more complex. Thus, to summarize, in order to assure adequate isolation of the controlled circuit from other electronic circuits, optical coupling was used. However, this necessitated that the triggering power for the thyristor come from a separate power source or from the controlled circuit. But recall that it was also desired that the thyristor be triggered at substantially zero voltage in the controlled circuit. However, the energy to trigger is not available from the controlled circuit at crossovers. These and other complications prevented prior art zero voltage switching, optically isolated relays from operating as efficiently as desired. For example, many prior "zero voltage switching" relays really switched only near, but never at, zero voltage.
It is, therefore, an object of this invention to provide a control circuit for a zero voltage switching thyristor that functions efficiently, provides true zero voltage switching and is amenable to optical isolation.